Semiconductor Electrodes I.I. Efficient Electroluminescence at ZnS Electrode in Aqueous Electrolytes

Electroluminescence (EL) on ZnS was studied in aqueous olutions containing various redox species. For species able to generate strongly oxidizing intermediates, such as peroxydisulfate or hydrogen peroxide, bright blue luminescence was observed uring cathodic polarization at potentials near to or negative of flatband potential (V ~B) of ZnS. For solutions con-taining supporting electrolyte alone at various pH's, no emission was detectable even at potentials 7V negative ofVrR. This suggests that minority carrier (hole) injection is responsible for the initiation of EL. The peak energy of the EL spectrum was much smaller (by 1 eV) than the bandgap of ZnS, suggesting that the radiative r combination is through intermediate lumi-nescent centers. Under steady-state conditions, the EL intensity was proportional to the square of the current, suggesting that EL intensity is dominated by the recombination f electron hole pairs at luminescent centers. In the early part of a potential pulse (especially the first pulse), the growth behavior of EL intensity was strongly affected by the electron trapping of the empty upper luminescent s ates. The location of the EL spectra depended on the current density and EL intensity; with increasing EL intensity, a significant blue shift of the emission peak was observed. These results suggest that the overall radiative recombination rate might be limited by electron transfer (through a tunneling mechanism) from the occ,upied upper luminescent s ates to the empty lower lufninescent states. An EL efficiency of 0.2 % can be achieved by operating at a current density of 25 mA/cm 2